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The influence of soil density and the character of radioactive 134Cs and 137Cs pollution's distribution on in situ measurements
Zhukouski A.1, Anshakou O.1, Nichyparchuk A.1, Marozik P.2, Kuten S.3
'SPE «АТОМТЕХ»,
Gikalo str., 5, Minsk 220005, Belarus
2International Sakharov Environmental Institute, Belarusian State University, Dolgobrodskaya str., 23, Minsk 220070, Belarus 3Institute for Nuclear Problems of Belarusian State University, Bobruiskaya str., 11, Minsk 220030, Belarus
Received 12.01.2018
Accepted for publication 19.02.2018
Abstract
Periodic radiation monitoring of soils today is a priority task not only for Belarus, but also for Japan, suffered by Fukushima nuclear power plant accident. Use of portable and light spectrometers with ability to perform in situ measurements makes it possible to quickly estimate specific activity of measured radionuclides with required accuracy in particular soil site. Basic information of a gamma radiation source (radionuclides content, effective radius of measurement area and thickness of contaminated layer) can be obtained directly during measurement. The purpose of this research is to test the feasibility of using algorithms for determination of specific activity and thickness of contaminated layer under conditions of soil measurement with variable density parameters and radiocesium distribution in soil.
Monte-Carlo simulating allowed to estimate the degree of deviation of the shape of simulated spectra obtained with the use of Monte-Carlo soil model with uniformly distributed radionuclide in it, and for the case when the radionuclide distribution by soil profile can be described by an exponential function. For these cases of natural distribution of radiocesium, the pulse-amplitude spectrum is formed by an effective thickness of the contaminated site, which contains more than 90 % of radionuclides.
The developed Monte-Carlo model of a probe and contaminated soil site allows to estimate the effect of the variability of soil density on the total count rate of the pulse-amplitude spectrum. As a result of theoretical estimations, the relationship between the effective radius of contaminated site is determined as a function of soil density.
Analysis of the influence of radial zones of the cylindrical gamma source on in situ gamma-spectrometer showed that the main contribution to the total count rate of the pulse-amplitude spectrum is made by the radial zone with radius of up to 40 cm from the center of the probe, regardless of the thickness of the contaminated layer in geometry «Probe is located on the soil surface». A small site facilitates the selection of measurement area of land with a sufficiently flat surface, which is desirable during surveying the territories, especially with complex terrain.
Keywords: in situ gamma-spectrometer, contaminated soil layer, non-uniform distribution, effective thickness, density of soil.
DOI: 10.21122/2220-9506-2018-9-1-40-47
Адрес для переписки:
Жуковский А.И.
НПУП «АТОМТЕХ»,
ул. Гикало, 5, 220005 Минск, Беларусь
e-mail: [email protected]
Address for correspondence:
Zhukouski A.
SPE «ATOMTEX»,
Gikalo str., 5, Minsk 220005, Belarus
e-mail: [email protected]
Для цитирования:
Zhukouski A., Anshakou O., Nichyparchuk A., Marozik P., Kuten S.. The influence of soil density and the character of radioactive 134Cs and 137Cs pollution's distribution on in situ measurements. Приборы и методы измерений. 2018. - Т. 9, № 1. С. 40-47. DOI: 10.21122/2220-9506-2018-9-1-40-47
For citation:
Zhukouski A., Anshakou O., Nichyparchuk A., Marozik P., Kuten S. The influence of soil density and the character of radioactive 134Cs and 137Cs pollution's distribution on in situ measurements. Devices and Methods of Measurements. 2018, vol. 9, no. 1, pp. 40-47. DOI: 10.21122/2220-9506-2018-9-1-40-47
Влияние плотности почвы и характера распределения радионуклидов 134Cs и 137Cs по профилю при in situ измерениях
Жуковский А.И.1, Аншаков О.М.1, Ничипорчук А.О.1, Морозик П.М.2, Кутень С.А.3
НПУП «АТОМТЕХ»,
ул. Гикало, 5, 220005 Минск, Беларусь
2Международный государственный экологический институт имени А.Д. Сахарова Белорусского государственного университета, ул. Долгобродская, 23, 220070Минск, Беларусь
3Институт ядерных проблем Белорусского государственного университета, ул. Бобруйская, 11, 220030 Минск, Беларусь
Поступила 12.01.2018 Принята к печати 19.02.2018
Проведение периодического радиационного мониторинга почв на сегодняшний день является одной из приоритетных задач для обеспечения радиационной безопасности не только в Беларуси, пострадавшей от Чернобыльской катастрофы, но и в Японии, территории которой подверглись радиоактивному загрязнению в результате аварии на АЭС Фукусима. Цель настоящей работы заключалась в проверке возможности применения разработанных на основе упрощенной («равномерной») модели алгоритмов определения активности и толщины загрязненного слоя в условиях радиометрии почвенного покрова с вариативными параметрами плотности и распределения радиоцезия по глубине.
Использование портативных in situ спектрометров позволяет оперативно оценить удельную активность контролируемых радионуклидов и плотность загрязнения почвы прямо на месте измерения и с необходимой точностью. Основная информация об источнике гамма-излучения (присутствующие радионуклиды, эффективный радиус участка и толщина загрязненного слоя) может быть получена непосредственно по результатам измерения аппаратурного спектра в сравнении с теоретическими (калибровочными) спектрами. Теоретические спектры рассчитывают путем имитационного моделирования процесса in situ измерений активности радионуклидов 134Cs и 137Cs, равномерно распределенных в однородной почвенной среде постоянной плотности. На практике следует учитывать приблизительность принятой модели измерений в отношении реального профиля заглубления радиоактивных загрязнений и изменчивости плотности исследуемых почв.
Анализ влияния радиальных зон цилиндрического источника на интегральную скорость счета спектрометра показал, что основной вклад вносят радиальные зоны в радиусе до 40 см от центра устройства детектирования при расположении его на поверхности почвы вне зависимости от толщины загрязненного слоя. Небольшая по площади зона влияния облегчает выбор контролируемых участков земли с достаточно плоской поверхностью, что желательно при обследовании территорий, особенно со сложным рельефом.
Ключевые слова: in situ гамма-спектрометр, загрязненный слой почвы, неравномерное распределение радионуклида, эффективная толщина, плотность почвы
DOI: 10.21122/2220-9506-2018-9-1-40-47
Адрес для переписки:
Жуковский А.И.
НПУП «АТОМТЕХ»,
ул. Гикало, 5, 220005 Минск, Беларусь
e-mail: [email protected]
Address for correspondence:
Zhukouski A.
SPE «ATOMTEX»,
Gikalo str., 5, Minsk 220005, Belarus
e-mail: [email protected]
Для цитирования:
Zhukouski A., Anshakou O., Nichyparchuk A., MarozikP., Kuten S.. The influence of soil density and the character of radioactive 134Cs and 137Cs pollution's distribution on in situ measurements. Приборы и методы измерений. 2018. - Т. 9, № 1. С. 40-47. DOI: 10.21122/2220-9506-2018-9-1-40-47
For citation:
Zhukouski A., Anshakou O., Nichyparchuk A., Marozik P., Kuten S. The influence of soil density and the character of radioactive 134Cs and 137Cs pollution's distribution on in situ measurements. Devices and Methods of Measurements. 2018, vol. 9, no. 1, pp. 40-47. DOI: 10.21122/2220-9506-2018-9-1-40-47
Introduction
The soil, together with atmosphere air and natural waters, is a main subject for radiation and environmental monitoring [1, 2]. Portable gamma spectrometers with the probe, located above the soil (usually at 1 m height) [3-6] or on its surface, are being developed for the purposes of immediate survey of soil radioactive contamination [3, 7]. An important condition for the reliability and validity of in situ techniques of radionuclides activity determination in soil is to take into account the effect of its density and thickness of contaminated layer, as well as the distribution profile of radioactive substances in soil [3, 7, 8].
The described in [7] principles of the analysis of experimental spectra allows to simultaneously obtain information on the content and thickness layer with gamma-emitting 134Cs and 137Cs nuclides, expecting their homogeneous distribution over the contaminated layer. The algorithm for determination of specific activity and thickness of the contaminated layer is based on comparative analysis of experimental and simulated spectra of cesium isotopes obtained for a scintillation gamma spectrometer [7]. Simulated spectra were calculated using the Monte-Carlo (MC) method for the NaI(Tl) detector (0 63 x 0 63 mm) in the model of radiometric measurements of the contaminated site of the soil, represented as a cylindrical source with uniformly distributed 134Cs and 137Cs isotopes. The elemental composition (the effective atomic number Zeff ~ 10) and the density (1.5 g/cm3) of the model soil are chosen with maximum correspondence to average parameters of soils of Tohoku region (Japan) and Gomel region (Belarus) [2, 9-11]. For typical soils in areas contaminated by the Chernobyl and Fukushima nuclear accidents, the Zeff values are almost the same in a wide range of densities from 1.0 to 1.8 g/cm3 [2, 9, 10].
The purpose of this research is the verification of the application of the algorithm, proposed in [7], for determination of specific activity and thickness of the contaminated layer under conditions of soil radiometry with variable density parameters and ra-diocesium distribution by soil profile.
Materials and methods
The researches were based on simulation of the radiometric MC model [7], developed according to the purpose of the work and parameters of the probe. MCNP 4B software was used for simulating [12]. The model of the measurement soil vol-
ume, as in [7], is a cylindrical source located in a semi-infinite isotropic environment with the radio-actively contaminated layer of the fixed thickness. Gamma radiation of contaminated soil site was registered by the detector located on the middle of the upper surface of a cylindrical source. The studied model, unlike the prototype, is not limited to one average value of the soil density and the requirement of uniform distribution of cesium radionuclides over a layer of fixed thickness and can be used to assess the uncertainty of measurement in situ with variable parameters.
The height of the cylindrical source d is equal to the thickness of contaminated layer. The field of-view of the probe characterizes the radius r. In MC modeling, the number of emitted particles with a given energy should be distributed uniformly over the cylinder. With an increase in the radius r, the fraction of the photons emitted from regions of measurement area located far from the detector (and also the simulation time) increases rapidly, but the chance of these photons to contribute to the energy distribution of the pulse-height spectrum (F8 tally) becomes less and less important. Therefore, when modeling an extended (including infinite) source, the size of the measurement area should be restricted by some effective radius reff at which the energy distribution of the pulse-amplitude spectrum I(E) will be close to experimental one for the real object.
To determine the effective radius is need to use acceptable relative deviation p. By definition, the quantity p is written in the form:
Ifep (to)- IFEP (r )
(1)
h
Ito)
were iFEp(r) - the count rate the range ± 3 a of the full energy peak (FEP) for the source of given radius r; Ifep (œ) - the count rate in the same range for the source of infinite radius.
Value of r can be considered as the effective radius r = rff of the source corresponding to the relative deviation p.
The areas behind the circle with radius r > r „
eff
make an insignificant contribution to the value IFEP(r), since the probability of a gamma-quantum reaching from them to the detector, even after scattering in the soil and air, is negligible. At p = 10 %, a source with a radius rejf provides 90 % of the total count rate in energy range from 50 to 3000 keV of simulated spectrum of contaminated with cesium ra-
dionuclides soil layer with thickness d in the definite geometry of measuring.
The value of the effective radius for a uniformly contaminated soil layer is mainly determined by its density and energy gamma radiation of the measured radionuclide, and weakly depends on the elemental composition of the substance [13, 14]. For radionuclide with several gamma-ray energies, the value r f
eJj
is set by the most intense one. For nearby energies of the main gamma-ray energies of 134Cs and 137Cs in the range 550-900 keV, the effective radius values will not differ significantly. In further analysis, an isotope 137Cs with gamma-ray energy of 662 keV was used to simplify the simulation in order to obtain simulated spectra.
In cases, observed in practice, the depth of distribution of radioactive pollution into the soil does not exceed 20-25 cm [2, 15, 16]. During natural migration, the content of radionuclides usually decreases in depends on depth [2, 15-17]. The process of radionuclides redistribution by profile depends on many factors: soil type and density, time since radioactive contamination; agricultural activities, quantity of clay content, etc. Over the time, the maximum content of radionuclides along the soil layers may vary inland. Even within a relatively small area, cesium isotopes can be concentrated both in comparatively thin (2-3 cm) and thick (over 10 cm) layers [17-19]. To take into account a non-uniformly distribution of radiocesium by soil profile, it is necessary to introduce a concept «effective depth» or «effective thickness» of the contaminated soil layer.
The non-uniformly distribution of radionuclides in the soil and the effective thickness of the contaminated soil layer
For soil model with non-uniformly distributed 137Cs isotope, only few cases from the set of possible radionuclide distributions along the profile were considered [11, 19]. Radionuclide content Q was distributed from the surface into the soil on depth up to 3 cm, up to 5 cm, up to 7 cm and up to 12 cm according with dependences presented in Table 1.
The situation when the layer contaminated with the radionuclide was located under a «clean» cover of the soil was also researched. In this case, a clean environment was modeled between probe and contaminated soil layer with a soil layer of a similar elemental composition 1, 2, and 3 cm thick, in which photon starts were not set.
Relative radionuclide content Q in the layer of the soil profile
Table 1 top and lower
The thickness of the soil layer with a non-uniform-ly distributed radionuclide The content of 137Cs in the top layer, (0-1 cm), % The content of 137Cs in the lower layer, % Exponential dependence of radionuclide content by soil profile
up to 3 cm 65.4 9.3 g=173.8e-°-977d
up to 5 cm 55.0 2.5 g=112.7e-°-757d
up to 7 cm 48.0 1.0 Q=87.19e-0630d
up to 12 cm 38.0 0.2 Q=58.60e-0-464d
The radiocesium experimental spectrum from contaminated soil during natural migration of the nuclide into the soil will differ in shape from the «equivalent» spectrum with uniform distribution of the nuclide in the soil layer of the same thickness. The MC simulating results showed that for simulated spectrum, which calculated for a non-uniformly distribution of the radionuclide in the soil layer to a defined depth (hereinafter, the spectrum Sun), some equivalent soil thickness with uniform radionuclide distribution can be found, in which an identical simulated spectrum is formed (hereinafter, the «equiva-
lent» spectrum Seq) (Figure 1).
Figure 1 - Spectrum Sun with non-uniformly distributed 137Cs and its «equivalent» spectrum Se with uniformly distributed 137Cs
The deviation of the FEP heights with the gamma-ray energy spectrum of 662 keV Sun relatively to the «equivalent» spectrum Seq, cited to mass unit considering the value of equivalent thickness of contaminated layer is presented in Table 2.
In the process of MC simulating of spectra, more than 90 % of photon starts were distributed in the thickness of the source commensurate to the thickness of «equivalent» spectra. Thus, more than 90 % of the experimental spectrum will be formed due to contaminated layer of effective thickness equal to thickness of uniformly contaminated soil
layer, which ensures the formation of an «equivalent» spectrum S .
A un
Table 2
Deviation of FEP height with gamma radiation energy of 662 keV spectra Sun relatively to «equivalent» spectra S
The thickness of the soil layer with a non-uniformly distributed 137Cs along the profile (Table 1), cm Thickness «clean» soil layer above the contaminated layer, cm The thickness of the soil layer with an uniform distribution 137Cs, cm The ratio of the FEP height, %
0 2 -5.2
3 1 4 2.3
2 6 10.1
0 2 3.1
5 1 5 -1.7
2 7 0.7
0 4 -9.1
7 1 6 0.9
2 8 3.3
0 4 -5.2
12 1 6 5.0
2 10 1.2
1.3 g/cm3. In real conditions, the typical range of soil density is in the range from 1.1 g/cm3 to 1.8 g/cm3 with a tendency for increase by depth [2, 16].
In order to evaluate the effect of changes in soil density in the range from 1.3 g/cm3 to 1.8 g/cm3 on the count rate IFEP in FEP region (±3a), simulated spectra of the 137Cs radionuclide for different thicknesses of the contaminated layer were estimated. Figure 2 shows the results of MC simulations showing the inverse dependence of the effective radius ref on the soil density for equal values of the parameter p.
The influence of natural distribution of radionuclides into the soil on shape and intensity of experimental spectrum can be considered as unissued component of the systematic error in measuring by the methodological properties. According the data in the Table 2, for in situ measurements in geometry «Probe is located on the soil surface» the indicated error will not exceed 10 %.
The effect of soil density on in situ measurements
The results presented above, as well as the results of the MC simulation in [7], were obtained with an average homogeneous medium density of
Figure 2 - Dependence effective radius vs. soil density
The component of the error of in situ measurements caused by the variability of the soil density, was estimated from simulated spectra calculated using the effective radius dependencies on the density in the range from 1.3 g/cm3 to 1.8 g/cm3 (see Figure 2). Table 3 shows the calculated data reflecting the nature of the change in the effective radius and count rate in FEP range ±3a depending on the soil density.
It was found that in the range of soil densities from 1.3 g/cm3 to 1.8 g/cm3, the count rate I^ in the FEP region ± 3a do not change by more than ± 5 % compared to ^p, obtained with an average density of1.5 g/cm3.
The density of the soil by profile ppr is not constant and, as a rule, varies in the direction of increase, depending on the distribution [16]. Using MC simu-
Table 3
Effective radius values and changing of count rate in FEP range, depending on soil density
The density of the soil, g/cm3 1 1.3 1.5 1.8 2.5
The effective radius values for the thickness of the contaminated soil layer d = 3 cm, cm 82 73 69 63 56
The deviation of the count rate I relatively to the ones at soil density of 1.5 g/cm3 (d = 3 cm), % -15.2 0.0 3.5 22.3
The effective radius values for the thickness of the contaminated soil layer d = 15 cm, cm 62 57 54 49 46
The deviation of the count rate I relatively to the ones at soil density of 1.5 g/cm3 (d = 15 cm), % -9.0 -3.1 0.0 2.3 5.8
4
lation, the variation of the count rate IFEP was evaluated as a function of the variety in soil density by profile. The soil density varied in the range from 1.1 g/cm3 to 1.8 g/cm3 in increments of 0.2 g/cm3 per one centimeter of the soil profile. The radionuclide was distributed non-uniformly to depth of up to 5 cm and up to 7 cm (Table 1) by soil profile, with «clean» soil layer 1 cm thick located between the probe and the contaminated soil layer (Figure 3).
Figure 3 - Monte-Carlo model of the probe and the soil with different density by profile. The «clean» 0-1 cm layer of soil was located between the probe and contaminated layer 1-7 cm.
Figure 4 shows the simulated spectra of Sev with the 137Cs distribution depth up to 5 cm and up to 7 cm, their «equivalent» spectra Sun and simulated spectra obtained using MC soil model with a different density profile p with «clean» top (0-1 cm) layer.
Figure 4 - Simulated spectra of the 137Cs radionuclide in the soil: a - the spectrum Sev (d = 5 cm) with «clean» soil layer, ppr ^ const; b - the spectrum Sv (d = 5 cm) with «clean» soil layer, ppr = const; c - the «equivalent» spectrum Sun (d = 5 cm), ppr = const; d - the spectrum Sev (d = 7 cm) with «clean» soil layer, ppr = const; e - the spectrum Sv (d = 7 cm) with «clean» soil layer, ppr ^ const; f - the «equivalent» spectrum Sun (d = 6 cm), ppr = const
The heights deviations of the FEP for contaminated layers up to 5 cm and up to 7 cm with a different soil density compared to simulated spectra from a source with a density fixed at the profile of 1.5 g/ cm3 are within ± 1 %.
Contributions of radial source zones to the total count rate of the pulse-amplitude spectrum
The results presented above show that in during radiation monitoring of soils in the measurement geometry «Probe is located on the soil surface», the area of the cylindrical source, which provides the formation of 90 % of the total count rate of the pulse-amplitude spectrum, is limited to a circle with a radius of no more than 1 m regardless of soil density and nature of distribution of radiocesium along the profile.
In assessment of the effect of the sections of a cylindrical gamma-radiation source on the total count rate, the effective volume of the soil was considered as a set of radial zones - one central cylinder with a diameter of 10 cm and six coaxial cylinders, nested inside each other with an internal r from 10
m
to 60 cm and external r = (r + A) cm radii, where
out v m / 7
A is the width of the coaxial cylinder equal to 10 cm. The height of the cylinders was equal to the thickness of the contaminated layer d. When simulated spectra were calculated, photon starts were distributed only in the investigated radial zone, which subsequently moved from the center to the edge of the field-of-view of the probe limited by the effective radius reff The spectra were calculated for the thickness of the contaminated layer d = 5 cm and d = 15 cm. The main contribution to the total count rate within a circle with a radius of reff regardless of the thickness of the contaminated source layer, provides the radial zones, located directly next to probe in a radius of 40 cm. Analysis of MC simulation results in the form of a distribution of the relative contributions of radial zones to the total count rate within the region limited by reff is shown in Figure 5.
Figure 5 - Relative contribution of radial zone to the total count rate
For practical implementation of in situ soil measurements, it is not difficult to choose a soil
area whose surface will fully correspond to the adopted model of a cylindrical gamma-radiation source with a sufficiently flat surface. Wherein, there is no need to take into account the influence of trees, structures and other objects of the environment located at a distance of more than 2 meters. In addition, non-uniform ground at a distance of more than 40 cm from the center of the detector will not have a significant effect on the results of in situ measurements of the specific activity of ra-diocesium.
Summary
The results of MC simulating showed the efficiency of in situ measurement of soil with undefined density parameters and distribution of radiocesium in depth, using the algorithm of determine the thickness of the contaminated layer and the effective volume, which is based on comparative analysis of the experimental spectrum and simulated spectra.
The FEP count rate varies within ± 5 % for the soil density range from 1.3 g/cm3 to 1.8 g/cm3 relatively to the count rate at soil density of 1.5 g/ cm3. This deviation (± 5 %) can be considered as the non-excluded systematic error of the in situ measurement in geometry «Probe is located on the soil surface».
According to results of MC simulating, the spectrum, obtained for 134Cs and 137Cs isotopes with arbitrary distribution in the soil, has coincidences in form and intensity simulated spectrum of the same radionuclides, uniformly distributed in a layer of some thickness. Formation of the experimental spectrum at in situ measurements of contaminated soils with 134Cs and 137Cs radionuclides with a natural distribution by profile is provided by an effective thickness layer, containing more than 90 % of radiocesium. The methodical error of determining the specific activity of 134Cs and 137Cs radionuclides in soil for the considered cases of natural distribution by profile is within ± 10 %.
The presented data of MC simulating for determining of radial zone contribution to the total count rate of simulated spectrum demonstrate that the main contribution is made by the radial zone with radius of up to 40 cm from the center of the probe regardless of the thickness of the contaminated layer. Such small site facilitates the selection of measurement areas of soil with enough flat surface, which is desirable for surveying areas, especially with complex terrain.
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